The present invention generally relates to a method for improving the stability of ink compositions, and more specifically to a process for increasing the stability and integrity of ink compositions containing non-ionic surfactants therein.
Substantial developments have been made in the field of electronic printing technology. A wide variety of highly efficient printing systems currently exist which are capable of dispensing ink in a rapid and accurate manner. Thermal inkjet systems are especially important in this regard. Printing systems using thermal inkjet technology basically involve a cartridge unit which includes at least one ink reservoir chamber in fluid communication with a substrate having a plurality of resistors thereon. Selective activation of the resistors causes thermal excitation of the ink and expulsion thereof from the ink cartridge. Thermal inkjet cartridge units currently in use contain either a single ink composition (e.g. black) or a plurality of different colored ink compositions, with each composition being retained in a separate compartment. Representative thermal inkjet systems are discussed in U.S. Pat. No. 4,500,895 to Buck et al., U.S. Pat. No. 4,794,409 to Cowger et al.; U.S. Pat. No. 4,509,062 to Low et al.; U.S. Pat. No. 4,929,969 to Morris; U.S. Pat. No. 4,771,295 to Baker et al.; and the Hewlett-Packard Journal, Vol. 39, No. 4 (August, 1988), all of which are incorporated herein by reference.
To obtain a maximum level of print quality using a thermal inkjet apparatus or other printing system, the ink composition being delivered must be carefully formulated. Specifically, the ink composition must be capable of producing a clear and distinct image with proper dispersion on the selected substrate (e.g. paper). Also, the ink composition must be able to produce an image which is characterized by an absence of mottling and the avoidance of color bleed in multi-color printing systems (e.g. the bleed of one color into another). The present invention involves the use of a particular group of surfactant compositions to achieve these goals. The selected surfactants are capable of improving the wettability and dispersion characteristics of the ink in which they are used. In addition, the surfactants discussed below are capable of controlling mottling and color bleed in multi-color printing systems.
However, the addition of surfactants to an ink composition can lower the cloud point of the composition and diminish its stability. The present invention involves a method for controlling (e.g. raising) the cloud point of an ink composition which contains the desired surfactant materials therein. The term "cloud point" as used herein (which directly relates to the stability and integrity of the ink composition) involves the temperature of the ink composition at which phase separation of the surfactant and solvent occurs (e.g. the temperature at which the surfactant is no longer soluble in the solvent). The cloud point temperature of an ink composition will limit the temperature range at which the composition can be used and stored. Once the cloud point temperature is reached, the ink will no longer contain a single set of properties and will instead consist of multiple phases with different characteristics. This condition results in an ink composition with a "cloudy" appearance. The use of an ink composition which has reached its cloud point temperature can result in a substantial deterioration in print quality. It is therefore desirable to formulate an ink composition with as high a cloud point as possible (e.g. preferably at least about 60.degree. C.).
As described below, the present invention involves an ink composition containing specific surfactants which is treated in a unique manner to control (e.g. raise) the cloud point of the ink. Accordingly, a surfactant-containing ink can be formulated which is characterized by (1) a high cloud point; and (2) the avoidance of problems associated with mottling, color bleed, and the like. The present invention therefore represents an advance in the art of ink production technology as discussed below.